Luigi Di Sarno

Approximate Method for Transverse Response Analysis of Partially Isolated Bridges

Current analysis procedures for seismically isolated bridges frequently use an equivalent (approximate) linearization approach to represent the response of nonlinear isolation/energy dissipation devices. Linearization allows standard linear elastic analysis methods, e.g., the response spectrum method, to be conveniently used for design purposes. The linearization approach is by nature an iterative method implying the need to repeatedly correct and analyze a numerical finite-element model. A further simplification could be achieved using closed form equations to represent (1) the structure displacement patterns and (2) the restoring forces from structural elements. The paper explores such a possibility with reference to partially isolated continuous bridges, i.e., bridges with isolation devices at piers and pinned supports at abutments. The role and effect of higher modes of vibration on the system response are discussed, and an approximate method is proposed to account for such effects. An improvement of the classical Jacobsen’s approximation for the effective viscous damping ratio is also proposed using the results of response history analyses. The latter are carried out on two-dimensional numerical models of five case studies, generated from a real existing bridge supposed to be isolated with friction pendulum devices. Comparison of approximate predictions with response history analysis results is presented and discussed. Nonlinear dynamic analyses of a three-dimensional numerical model of the existing bridge were also carried out for comparison purposes.

Experimental Assessment of Seismic Pile-Soil Interaction

Physical modeling has long been established as a powerful tool for studying seismic pile-soil-superstructure interaction. This chapter presents a series of 1-g shaking table tests aiming at clarifying fundamental aspects of kinematic and inertial interaction effects on pile-supported systems. Pile models in layered sand deposits were built in the laboratory and subjected to a wide set of earthquake motions. The piles were densely instrumented with accelerometers and strain gauges; therefore, earthquake response, including bending strains along their length, could be measured directly. Certain broad conclusions on kinematic and inertial SSI effects on this type of systems are drawn.

Seismic behaviour of perimeter and spatial steel frames

The present study deals with the seismic performance of partial perimeter and spatial moment resisting frames (MRFs) for low-to-medium rise buildings. It seeks to establish perimeter configuration systems and hence the lack of redundancy can detrimentally affect the seismic response of framed buildings. The paper tackles this key issue by com-paring the performance of a set of perimeter and spatial MRFs, which were “consistently designed”. The starting point is the set of low-(three-storey) and medium-rise (nine-storey) perimeter frames designed within the SAC Steel Project for the Los Angeles, Seattle and Boston seismic zones. Extensive design analyses (static and multi-modal) of the perimeter frame buildings and consistent design of spatial frame systems, as an alternative to the perimeter configuration, were conducted within this analytical study. The objectives of the consistent design are two-fold, i.e. obtaining fundamental periods similar to those of the perimeter frames, i.e. same lateral stiffness under design horizon-tal loads, and supplying similar yield strength. The seismic behaviour of perimeter and spatial configuration structures was evaluated by means of push-over non-linear static analyses and inelastic dynamic analyses (non linear time histories). Comparisons be-tween analysis results were developed in a well defined framework since a clear scheme to define and evaluate relevant limit states is suggested. The failure modes, either local or global, were computed and correlated to design choices, particularly those concerning the strength requirements (column overstrength factors) and stiffness (elastic stability indexes). The inelastic response exhibited by the sample MRFs under severe ground motions was assessed in a detailed fashion. Conclusions are drawn in terms of local and global performance, namely global and inter-storey drifts, beam and column plas-tic rotations, hysteretic energy. The finding is that the seismic response of perimeter and spatial MRFs is fairly similar. Therefore, an equivalent behaviour between the two configurations can be obtained if the design is “consistent”.

Intercontinental Hybrid Simulation for the Assessment of a Three-Span R/C Highway Overpass

ABSTRACT This paper presents hybrid simulations of a three-span R/C bridge among EU, US, and Canada. The tests involved partners located on both sides of the Atlantic with each one assigned a numerical or a physical module of the substructured bridge. Despite the network latency in linking remote sites located on the two sides of the Atlantic the intercontinental hybrid simulation was accomplished and repeated successfully, highlighting the efficiency, and repetitiveness of the approach. Adaptations, challenges, and limitations are discussed, focusing on the implications of network communication latency, the insensitivity of the sub-structuring arrangement, and the accuracy of the results obtained.

Assessment of the Seismic Behaviour of a Retrofitted Old R.C. Highway Bridge Through PsD Testing

The RETRO project aims at studying the seismic behaviour of existing reinforced concrete (RC) bridges and the effectiveness of innovative retrofitting systems. A typical as-built RC highway viaduct, designed primarily for gravity loads, has been analysed experimentally. The objective of the laboratory test program is twofold: (1) improve the knowledge of the non-linear behaviour of RC framed piers without seismic detailing and (2) assess the effectiveness of seismic isolation systems as a structural mitigation measure. Two large scale framed piers with two and three levels were re-designed and tested using the PsD method with hybrid (analytical and experimental) simulation. Two test configurations were considered: (1) the as-built bridge layout and (2) the viaduct retrofitted by means of Friction Pendulm (FP) isolators. The seismic performance evaluation was carried out at two limit states, i.e. at serviceability and ultimate limit states. The experimental tests stressed the high structural vulnerability of the viaduct, confirmed by the extensive shear damage in the transverse beams and the fix-end rotation effects generated by the bond slip of plain steel bars in the columns embedded in the foundation. The effectiveness of the isolation system with FP devices was also demonstrated. Nevertheless, the friction of the FP devices is a critical response parameter which may vary significantly because of the pressure due to vertical loads, strong motion velocity and temperature. The reliable evaluation of the friction parameter is of paramount importance to prevent the onset of damage within the framed piers.

Refined and Simplified Numerical Models of an Isolated Old Highway Bridge for PsD Testing

RETRO’ project aims at studying the seismic behaviour of existing reinforced concrete (RC) bridges and the effectiveness of retrofitting systems based on seismic isolation of the deck of the viaduct. The research program focuses on a typical non-compliant bridge system for earthquake loading, designed for gravity loads only. The prototype structure is the Rio Torto bridge system, which is located in a region of medium seismic hazard in Italy. The seismic vulnerability of the as-built framed pier bridge is first assessed. A typical seismic isolation system, employing slide spherical bearings, is then designed as a passive control retrofitting measure. The present chapter discusses the non-linear response of the Rio-Torto viaduct in the “as-built” and “isolated” configurations. The seismic performance assessment is carried out by utilizing refined non-linear structural models implemented in an advanced and reliable computer platform. The earthquake behaviour of refined models used for the sample structure accounts for non-linear phenomena of the viaduct, e.g. strain penetration of plain bars, shear deformation of transverse beams, flexural deformations in columns and beams. The finite element models are calibrated on the basis of experimental tests results. The assessment of the seismic response system is investigated in terms of local and global response parameters. In addition, the effectiveness of the isolation systems used as a retrofitting system is also investigated numerically. The outcomes of the comprehensive nonlinear analyses are used to simulate the seismic response of the viaduct in the as-built and isolated configurations during Pseudo-dynamic testing, which is illustrated in a companion chapter.

Seismic Performance Assessment of Existing Steel Buildings: A Case Study

Numerous existing steel framed buildings located in earthquake prone regions world-wide were designed without seismic provisions. Slender beam-columns, as well as non-ductile beam-to-column connections have been employed for multi-storey moment-resisting frames (MRFs) built before the 80’s. Thus, widespread damage due to brittle failure has been commonly observed in the past earthquakes for steel MRFs. A recent post-earthquake survey carried out in the aftermath of the 2016-2017 Central Italy seismic swarm has pointed out that steel structures may survive the shaking caused by several main-shocks and strong aftershocks without collapsing. Inevitably, significant lateral deformations are experienced, and, in turn, non-structural components are severely damaged thus inhibiting the use of the steel building structures. The present papers illustrates the outcomes of a recent preliminary numerical study carried out for the case of a steel MRF building located in Amatrice, Central Italy, which experienced a series of ground motion excitations suffering significant damage to the masonry infills without collapsing. A refined numerical model of the sample structure has been developed on the basis of the data collected on site. Given the lack of design drawings, the structure has been re-designed in compliance with the Italian regulations imposed at the time of construction employing the allowable stress method. The earthquake performance of the case study MRF has been then investigated through advanced nonlinear dynamic analyses and its structural performance has been evaluated according to Eurocode 8-Part 3 for existing buildings. The reliability of the codified approaches has been evaluated and possible improvements emphasized.

Structural health monitoring of existing bridges in earthquake prone areas: Laboratory validation

Road infrastructures and particularly bridges can suffer structural damage due to earthquakes threatening the efficiency of the transportation network and the possibility to ensure prompt rescue operation. This particularly applies to existing bridges, most of which have been designed and built according to outdated codes. Structural Health Monitoring (SHM) systems can support the prompt assessment of bridges after seismic events. However, reliability of modal based damage detection currently depends on the accuracy of modal parameter estimates automatically obtained from the analysis of the operational response of the monitored structure, and on the capability of the measurement system to resolve low amplitude as well as strong motions, eventually associated to saturation of sensors. In the present paper, the performance of a modal-based SHM system for existing bridges in seismic areas is assessed by shaking table tests on a 1:3 scale single span bridge representative of existing highway bridges built in the 60s in Italy. Results show that hidden damage can be identified on a remote basis, thus demonstrating the interesting applicative perspectives of modal based SHM for fast assessment of existing bridges in the early earthquake aftershock. The resilience to earthquake shaking of the SHM system has been also assessed. Finally, specific data processing procedures for earthquake response data are tested and compared with the results of laboratory measurements.

SEISMIC FRAGILITY OF FREESTANDING BUILDINGS CONTENTS MODELLED AS RIGID BLOCKS

The loss of functionality of health care facilities, which should be guaranteed particularly in the aftermath of moderate-to-severe earthquake ground motions, is typically caused by damage to nonstructural elements, such as freestanding cabinets. The assessment of the seismic fragility of such components assumes a key role in the evaluation of the performance of a healthcare facility. The present work is aimed to assess the adequacy of the rigid block modeling approach in predicting the seismic response of freestanding nonstructural components with rockingdominated response. The outcomes of the numerical analyses show that the considered modeling technique can provide a reliable prediction of the occurrence of rocking mechanism and predict the occurrence of the overturning. In particular, the overturning PFA is slightly underestimated in case a 1.0 coefficient of restitution is considered. But the question then arises as to which intensity measure (IM) is well correlated to the seismic performance of rigid blocks. A fragility study on a number of rigid blocks is therefore conducted in the present paper. Comprehensive incremental dynamic analyses on different rigid blocks highlight that the dimensionless intensity measure PGA/(g tanα) is an efficient intensity measures to predict rocking occurrence in a generic rigid block. The intensity measure pPGV/ (g tanα) is the most efficient one only for large, say R larger than 2.0 m, rigid blocks. Very small, say R<1.0 m, rigid blocks tend to overturn as soon as they start rocking and are therefore ‘‘PGAdominated’’. PGA/(g tanα) is therefore more efficient for such blocks. The use of these intensity measures allows assessing a unique fragility curve for rigid blocks characterized by different geometries, which may serve as a simple tool for the estimation of the damage occurred in rigid blocks after earthquakes. 2926 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 2926-2936

NUMERICAL SIMULATION OF SOIL-STRUCTURE INTERACTION: A PARAMETRIC STUDY

Soil Structure Interaction (SSI) is a complex phenomenon that may radically change the earthquake response of structural systems, as consequence of the variation of the natural frequency and the damping ratio. One of the most effective means for evaluating SSI is the use of physical models. In this study the physical model considered is formed by an oscillator founded on a group of piles embedded in a horizontally layered deposit of dry sand. The benchmark experimental campaign was carried out at the Bristol Laboratory for Advanced Dynamics Engineering (BLADE) at the University of Bristol (UK), financed by the Seismic Engineering Research Infrastructures for European Synergies (SERIES). An accurate parametric numerical study is performed and the results are discussed. The study investigates the effects of the dynamic properties of the oscillator on the period elongation and piles response. By means of advanced numerical analyses the outcomes of the present work provide insights into the quantitative evaluation of period elongation and the strength of inertial contribute to the bending moment at the pile head, when the system approaches resonance conditions. 3568 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 3568-3577